In holographic data storage digital data are stored by recording the interference pattern produced by the superposition of two coherent laser beams, where one beam, the so-called ‘object beam’, is modulated by a spatial light modulator and carries the information to be recorded. The second beam serves as a reference beam. The interference pattern leads to modifications of specific properties of the storage material, which depend on the local intensity of the interference pattern. Reading of a recorded hologram is performed by illuminating the hologram with the reference beam using the same conditions as during recording. This results in the reconstruction of the recorded object beam.
One advantage of holographic data storage is an increased data capacity. Contrary to conventional optical storage media, the volume of the holographic storage medium is used for storing information, not just a few layers. One further advantage of holographic data storage is the possibility to store multiple data in the same volume, e.g. by changing the angle between the two beams or by using shift multiplexing, etc. Furthermore, instead of storing single bits, data are stored as data pages. Typically a data page consists of a matrix of light-dark-patterns, i.e. a two dimensional binary array or an array of grey values, which code multiple bits. This allows to achieve increased data rates in addition to the increased storage density. The data page is imprinted onto the object beam by the spatial light modulator (SLM) and detected with a detector array.
In WO2006/003077 a 12f reflection type coaxial holographic storage arrangement with three confocally arranged Fourier planes is shown. In this arrangement the object beam and the reference beams are coupled in and out at the first and third Fourier planes, respectively. The reference beams are small spots in these planes. More precisely, they form diffraction patterns, similar to the Airy pattern. This arrangement is a so-called common aperture arrangement, because at the object plane and the image plane the object beam and the reference beams fill out the same area of the aperture. The beams fill out the entire aperture of the objectives. The disclosed arrangement allows to apply shift multiplexing, reference scanning multiplexing, phase coded multiplexing, or a combination of these multiplexing schemes. The reference beams are a pair (or pairs of) half cone shaped beams. The tips of the pair or pairs of half cone shaped reference beams form two lines along a diameter at the Fourier planes of the object beam.
Theoretically, for infinite holograms the shift selectivity curve is a sinc(x) like function. See, for example, G. Barbastathis et al.: “Shift multiplexing with spherical reference waves”, Appl. Opt. 35, pp 2403-2417. At the so-called Bragg distances the diffraction efficiencies of the shifted hologram are zero. In WO2006/003077 the distances between the tips of the reference beams along the two lines correspond to these Bragg distances. The interhologram crosstalk between the multiplexed holograms in theory is negligible at the Bragg distances. Assuming infinite diameter holograms there are selective and non-selective directions for the shift multiplexing. See again the article of G. Barbastathis et al. The selective direction is the direction of the line formed by the tips of the reference beams. In the so-called non-selective direction, which is orthogonal to the selective direction in the plane of the holograms, the shift distance is infinite. However, in a real storage system the volume of the hologram is finite. Practically, the order of magnitude of the hologram volume is about (0.4-0.6)×(0.4-0.6)×(0.2-0.6) mm3. Detailed investigations have shown that there are large discrepancies between the shift selectivity curves of infinite and finite holograms. There are no Bragg distances in case of finite volume holograms. See Z. Karpati et al.: “Shift Selectivity Calculation for Finite Volume Holograms with Half-Cone Reference Beams”, Jap. J. Appl. Phys., Vol. 45 (2006), pp 1288-1289. Using finite volume holograms the order of magnitude of the selectivity is similar in both directions. See, for example, Z. Karpati et al.: “Selectivity and tolerance calculations with half-cone reference beam in volume holographic storage”, J. Mod. Opt., Vol. 53 (2006), pp 2067-2088. The presence of selectivity in both directions allows two-dimensional multiplexing. A problem is that the interhologram cross-talk is too high in the non-selective direction. This limits the achievable number of multiplexed holograms in this direction, and as a consequence limits the total capacity of the holographic storage medium.
In order to obtain an improved selectivity, in the not yet published European Patent Application EP 06122233.7 an apparatus for reading from and/or writing to a reflection-type holographic storage medium with a coaxial arrangement of three or more reference beams and an object beam or a reconstructed object beam is described. In this apparatus the reference beams are arranged on a circle or an ellipse around the object beam in a Fourier plane of the apparatus. In order to separate a reconstructed object beam from the reflected reference beams an outcoupling filter is used, which blocks the reflected reference beams and passes the reconstructed object beam through a central aperture.
The main advantage of the various coaxial holographic storage systems is their insensitivity against environmental disturbances, because the object and reference beams propagate along the same optical path. Use of a reflection type holographic storage medium allows to reduce the size of the system compared to the size of a system for a transmission type holographic storage medium, as all optical elements are arranged on the same side of the holographic storage medium. Furthermore, no additional hardware is necessary for shift multiplexing. A precise movement of the holographic storage medium is sufficient, which can easily be realized by rotating the holographic storage medium.
However, a big challenge for the reflection type coaxial systems is to increase the Signal to Noise Ratio (SNR) by attenuation of the different noises propagating on the same axis as the reconstructed object beam. Because of the small diffraction efficiency of the multiplexed holograms, the order of magnitude of the required attenuation is about 10−4 or 10−5.